The purpose of this study is twofold 1) to confirm and extend data suggesting that recreational MDMA users incur damage to central serotonin neurons; and 2) to further probe for functional consequences of MDMA exposure. This research will test the hypothesis that MDMA neurotoxicity generalized to humans has functional consequences. The long-term goals of this project are to better delineate the public health risks of recreational MDMA use, and to utilize this unique population of individuals to further elucidate the role of brain serotonin in health and disease.